Separation of Aspirin from Acetaminophen and Caffeine in an Over-the-Counter Analgesic Tablet A Solid-Phase Extraction Method John P. Williams and Kevin J. west' University of Wisconsin-Whitewater, Whitewater, WI 53190 Karen L. Erickson Indiana University-Purdue University at Fort Wayne, Fort Wayne, IN 46805 Consumer products have been used over the years in a variety of laboratory experiments to expose students to analysis involving samples that are not made by the instructor ( I ) . Some of the methods rely on simultaneous spectrophotometricanalysis (2)to quantitate active ingredients. Analgesic tablets are commonly chosen for such analysis, especially those containing aspirin, caffeine,and acetamino~hen.The analvsis of these analgesic tablets usually reitires the extraction of one compon&t from the other two wm~onents.These extractions involve the use of relatively large quantities, greater than 600 mL per student, of a halogenated organic solvent such as methylene chloride(3).Disposal of such solvents can be a considerable burden upon a department. Additionally, the pedagogical value of such a procedure needs to be considered relative to the exposure of students to such solvents. An alternative extraction procedure was sought that would use smaller volumes of "safer" solvents in performing the extraction and allow the use of simultaneous spectrophotometric analysis in the determination. The method selected for study was solid-phase extraction that is attractive because small volumes of solvent are used in the extraction and with proper selection of the solid phase used for the extraction, the solvents may be aqueous solutions or methanol. Experimental A 1M solution of sodium acetate was made by dissolving the required amount of reagent grade sodium acetate in distilled water and diluting to volume. Sodium acetate solutions having concentrations of 0.50 M and 0.01 M were made by dilution of the 1M sodium acetate solution with distilled water. A 1M acetic acid solution was made by dilution of glacial acetic acid with the resulting solution being further diluted to make a solution that was 0.20 M acetic acid. Standard solutions of acetaminophen (4acetamidophenol) and caffeine were prepared by dissolving ac-
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curately weighed amounts of the solid reagent in 0.01 M sodium acetate solution and adjusting the pH to a value of 9 with 0.1 M sodium hydroxide. Standard solutions of aspirin (acetylsalicylicacid) were prepared by dissolving accurately weighed amounts of the solid reagent in 2-3 mL of 0.5 M sodium hydroxide to hydrolyze the acetylsalicylic acid to the salicylate ion and diluting to the desired volume with 0.01 M sodium acetate solution. The pH of the resulting solution was found to have a value of 9. The hydrolysis of the aspirin will he discussed in detail below. Each standard solution had a concentration between 1.00 and 2.00 mg/mL of the standard reagent. All of the solid reagents are available from Aldrich Chemical Co. A solution of the analgesic tablet, Excedrin Extra Strength in this study, was prepared by dissolving a previously weighed tablet in 10 mL of 0.5 M sodium hydroxide solution with heating for 10 min on a hotplate, followed by 10 min of cooling. This solution was then quantitatively transferred to a 50-mL volumetric flask and diluted to volume with the 0.01 M sodium acetate solution. A 10-mL aliquot of the sample solution was then further diluted to 50 mL in a volumetric flask with the 0.01 M sodium acetate solution. The tablet weight was recorded to 0.1 mg from an analytical balance. The extraction of the aspirin from the solution of the analgesic tablet used a 3-mL quaternary amine solid phase extraction column (J.T. Baker Co. 7091-03) and a Baker SPE-10 vacuum manifold with a vacuum gauge available from J. T. Baker Co. The vacuum manifold was operated at a vacuum of 5 in. of mercury (in. Hg) on the manifold gauge. The quaternary amine column was conditioned by aspirating3 mL of reagent grade methanol, 3 mL of 0.20 M acetic acid, 3 mL of 1M sodium acetate solution, and 3 mL of 0.01 M sodium acetate solution through the column and to waste. The column was not allowed to dry between the solution additions. AO.lOO mL (100 pL) aliquot of the sample solution was placed on the column along with a 0.500 mL (500 pL)aliquot of the 0.01 M sodium acetate solution. The vacuum was applied and when this volume had been
Volume 69 Number 8 August 1992
669
aspirated onto the column, the column was rinsed with two 1-mL portions of the 0.01 M sodium acetate solution. The column was then allowed to dry. The eluant from the wlumn was collected directly into a 10-mL volumetric flask and contains the acetaminophen and caffeine. The column was then moved to another port and rinsed with five 1-mL aliquots of the 0.5 M sodium acetate solution. This eluant was also collected directly into a 10-mL volumetric flask and contains the aspirin. The contents of the two volumetric flasks were then diluted to volume with distilled water. Replicate extractions were performed on the same extraction column starting with the column conditioning step. All solutions were applied to the extraction column using loo-, 500-, and 1000-LEppendorf pipets as appropriate. Ultraviolet spectrophotometry was used to quantitate the aspirin, acetaminophen, and caffeine solutions. A 0.100-mL aliquot of each standard solution was mixed with 2.5 mL of the 0.01 M sodium acetate solution and 5.0 mL of the 0.5 M sodium acetate solutionin separate 10-mL volumetric flasks and each was diluted to volume with distilled water. The extracted solutions were used as obtained. The ultraviolet spectrum of each standard and each sample solution was obtained from 200-350 nm using a Perkin-Elmer Lambda 4B spectrophotometer with 1-cm quartz cuvets. The absorbance was measured for each solution a t the wavelength maximum for each analyte: 292 n m for aspirin, 242 n m for acetaminophen, and 272 nm for caffeine. Discussion The extraction procedure reported here utilizes the quaternam amine extraction column in an anion extraction mode. ?'his mode was selected to take advantage of the fact that asnirin is an anion a t DH 9 whereas the acetaminophen &d caffeine are not akons a t this pH. The aspirin, a s the anion. is thus extracted from a low ionic stren&h acetate ion solution onto the extraction column while ;he acetamino~henand caffeine Dass through the column. The aspirin anion can then be removed &the extraction column using an acetate ion solution of higher ionic strength. Since aspirin is susceptible to hydroly& to the salicyiate anion in alkaline solutiun, it was decided to hvdrolvze the aspirin prior to separation. An alkaline solution of salicylic acid was compared to the aspirin solution used in the separation. It was found that the absorption spectrum of the salicylate ion matched the absorption spectrum of the aspirin solution. However, the intensity of the salicylate ion absorption was larger than the intensity of the aspirin solution. The hydrolysis of the aspirin prior to separation thus removes a potential source of error due to the slow hvdrolvsis - " of asnirin and increases the sensitivitv of the aspirin determgation. The hydrolysis of aspirin ioes not require additional calculations a s the mass of aspirin is determined prior to hydrolysis for the standard solution. The same holds true for the aspirin in the analgesic tablet provided it does not contain salicylic acid in the formulation. The amount of salicylate ion formed in either case is dependent upon the amount of aspirin present prior to hydrolysis. The presence of the acid wash in the column preparation was found to he necessary to remove stearic acid that was present in the tablets as an inactive ingredient. The stearic acid also was converted to an anion in the sample solution and was extracted onto the ion-exchange column but was not removed during the column preparation. The addition of the acetic acid wash removed the stearate ion from the ion-exchange column by converting it back to stearic acid.
670
Journal of Chemical Education
Comparison of Typical Results with Analgesic Label Amounts Label Amounts Active Ingredient
Mass per Tablet 250 mg
Aspirin Acetaminophen Caffeine
250 rng 65 ma
Analysis Results Mass in mg
Tablet 1 Aspirin 250 Aceta251 minophen 62 Caffeine Tablet 2 Aspirin Acetaminophen Caffeine
Ave.
s(Rel.) Relative Error
260 258
254 259
255 256
5 (2%) 2% 4(2%) 2%
63
63
63
0.7(1%)-3%
Ave.
s(Rel.) Relative Error 4 (2%) -1% 1 (0.4%p.4%
Mass in rng 249 250
243 248
250 247
247 249
62
62
61
62
0.7(1%)4%
The absorbance values obtained from the ultraviolet spectrophotometric measurements were converted to concentratlons using Heer's law. The aspirin determination of Beer's law while the detcrmiused the sinele anolication ~ - - ~ - ~ . ~ * nation of acetaminophen and caffeine used the additive Beer's law to arrive at two equations that were solved simultaneously (4).Under the conditions used, aspirin was found to have an absorotivitv of 20 mWmecm at 295 nm. 65 Acetaminophen and faffeine had absorptivities mUmecm and 17 mUmecm.. resoectivelv. ".at 242 nm and 16 mI;/mgan and 51 rnCrngan, respectively, a t 272 nm. The concentrations were then converted to masses of each active ingredient by taking into account the dilutions performed upon the samnle solutions. TvDical results for the analysis two tablets of the same anhgesic are shown in the table. In ~eneral,relative errors were less than 5% for the calculated masses of each active ingredient with relative standard deviations less than 3%. The extraction procedure reported here addresses the concerns of using large volumes of halogenated organic solvents. The solvents used are methanol and aqueous sodium acetate solutions with the total solvent volume reduced to 200 m i perstudent. The procedure also allow* fir the aoolication of Beer's law in hoth the sinele and additive f&s. An additional benefit was encountkd with the reduced time required to perform the extraction. T h ~ e replicate extractions can be done with solid-phase extraction in about the same time required to perform a single traditional extraction, thus allowing the estimation of precision for the analysis.
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